The long-term goal of this proposal is to help clarify how the lungs regulate the peptide hormone composition of systemic arterial blood. Emphasis is placed on identifying and characterizing enzymes that are disposed on or near the luminal surface of pulmonary microvascular endothelium and that interact with peptide hormones and prohormones of central venous blood. Special focus is place on aminopeptidase p (AmP) aims of this proposal are to 1) improve methods of purifying AmP and extend these methods to enable purification of insoluble forms of the enzyme, 2) characterize pure forms of AmP in terms of substrate affinities and substrate subsite preferences, 3) help clarify the primary and secondary structures of AmP by amino acid sequencing and mass spectroscopy, and 4) characterize AmP immunologically (including immunocytochemical localizations) using monoclonal antibodies. By so proceeding, it is expected that clarification will be obtained for a previously undocumented means by which the lungs prevent the entry of bradykinin into systemic arterial blood. In addition to the intrinsic interest of learning how lungs regulate at least one hormone system, the results are likely to have clinical implications. The lungs are know to contain another enzyme, angiotensin converting enzyme (ACE), that is capable of inactivating bradykinin. Inhibitors of ACE are in wide use for treatment of high blood pressure and congestive heart failure. By clarifying interactions of circulating bradykinin with pulmonary aminopeptidase P, new insights may be gained into how systemic arterial blood concentrations of bradykinin are controlled when ACE is fully inhibited. In addition, preliminary data suggest that AmP may also degrade head peptide and antiarrhythmic peptide. Thus, insights may be gained into mechanisms by which AmP regulates three different peptide hormone systems; one that affects vascular tone, another that stimulates exocrine pancreatic secretion, and another that affects cardiac rhythm.